1. Field of the Invention
The present invention relates to an image forming apparatus for development by using a two-component developing agent in accordance with electrophotography and, more particularly, to an image forming apparatus which employs a recycling scheme of recovering and reusing transfer residue toner and a convey control method for recycle toner.
2. Description of the Related Art
According to an example of an image forming process of forming an image by electrophotography, an electrostatic latent image is formed on a latent image carrier such as a photosensitive body. The formed electrostatic latent image is developed by a developing means to form a toner image on the latent image carrier. The formed toner image is transferred onto transfer paper by a transfer means. The transferred toner image is fixed on the transfer paper by a fixing means. Thus, an image is formed on the transfer paper. According to another example, a toner image on an image forming body such as a photosensitive body is transferred onto an intermediate transfer body serving as an image carrier. The toner image is then transferred from the intermediate transfer body onto transfer paper by a transfer means, and is fixed. Thus, an image is formed on the transfer paper.
In the developing step of an image forming process, development using a two-component developing agent containing nonmagnetic toner and a magnetic carrier is often employed, and a DC-bias developing bias voltage is applied.
In development using the two-component developing agent, since only the toner is consumed by development, an appropriate amount of new toner corresponding to the consumed amount must be replenished. Thus, toner replenishment is performed.
The toner image formed on the latent image carrier is transferred onto the transfer paper or intermediate transfer body by the transfer means. At this time, the transfer residue toner which is not transferred but left on the latent image carrier is cleaned by a cleaning means. A recycling scheme is employed in which the toner cleaned by the cleaning means and recovered in a cleaning unit is conveyed to a developing unit and is reused. When the recycling scheme is employed, a waste toner box for collecting the recovered toner to dispose it is not necessary. In this manner, the recycling scheme can be regarded as a scheme that reduces waste and is accordingly friendly to the environment.
A dynamic torque required for conveying the toner recovered by the recycling means to the developing unit is as small as about {fraction (1/10)} that required for driving the latent image carrier and fixing unit. To reduce the cost and to save the space, the recycling means is driven by the same driving source as for the latent image carrier or fixing unit.
In the cleaning unit, the toner (recycle toner) recovered by the cleaning means has been conveyed after it is scraped by a cleaning blade or the like. The recycle toner has accordingly been subjected to a mechanical stress. When the recycle toner is compared to new toner, it has lower flowability, it is harder to mix with the carrier, and its electrostatic properties also degrade. When the recycle toner is developed again, it tends to pose a problem such as fogging or character thickening. This phenomenon does not substantially pose any problem when the ratio of the recycle toner in the developing agent is low, but does when the ratio is high.
When toner having a small particle size or toner manufactured by a polymerization method and having a sharp particle size distribution is used, an image quality such as resolution, tone, character reproducibility, and the like is high. Therefore, when the recycle toner described above is used, the above problem is obvious.
This problem of degradation in image quality occurs as follows. When the printing ratio of an image formation pattern to be printed continuously is low, the amount of new toner to be replenished becomes small, and accordingly the proportion of the recycle toner in the developing agent increases. This causes obvious degradation in image quality.
It is known that the problem of degradation in image quality tends to occur when the image forming apparatus has been kept stopped for a long period of time, e.g., for 2 h or more, and then several hundred prints are output. This is because the charges of the recycle toner damaged due to the long-time stop are decreased.
To solve this phenomenon, a single driving source may be employed as a driving source for the recycling means, or the amount of recycle toner to be returned to the developing unit may be controlled in accordance with the printing ratio of an image formation pattern. Alternatively, while several hundred prints are output after long-time stop, the amount of recycle toner to be returned to the developing unit may be controlled. If, however, a single driving source is provided to the recycling means in order to control the amount of recycle toner, cost reduction and space saving cannot be achieved.